Transparent oxide semiconductor thin film transistor

ABSTRACT

This invention relates to novel, transparent oxide semiconductor thin film transistors (TFT&#39;s) and a process for making them.

FIELD OF THE INVENTION

The present invention relates to a transistor fabricated with atransparent oxide semiconductor selected from the group consisting ofzinc oxide, indium oxide, tin oxide, and cadmium oxide deposited withoutthe intentional incorporation of additional doping elements and theprocess for deposition of the oxide semiconductors. Transparent oxidesemiconductors are useful in fabrication of transparent thin filmtransistors. Transparent transistors can be used to control pixels in adisplay. By being transparent, the transistor may not significantlyreduce the active area of the pixel.

TECHNICAL BACKGROUND

Fortunato et al (Materials Research Society Symposium Proceedings (2001)666) described zinc oxide films containing aluminum deposited onpolyester by radio-frequency magnetron sputtering.

Japanese Patent Application 2002076356 A describes a channel layer madeof zinc oxide and doped with transition metals.

Goodman (U.S. Pat. No. 4,204,217 A) discloses a liquid crystaltransistor.

Ohya et al (Japanese Journal of Applied Physics, Part 1 (January 2001)vol 40, no. 1, p 297-8 disclose a thin film transistor of ZnO fabricatedby chemical solution deposition.

Maniv et al (J. Vac. Sci Technol., A (1983), 1(3), 1370-5 describeconducting zinc oxide films prepared by modified reactive planarmagnetron sputtering.

Giancaterina et al (Surface and Coatings Technology (2001) 138(1), 84-94describe zinc oxide coatings deposited by radio frequency magnetronsputtering.

Seager et al. (Appl. Phys. Lett. 68, 2660-2662, 1996) describe using theelectric field emanating from a ferroelectric insulator to control ormodulate resistance in a conducting film of ZnO:Al or ZnO:In.

Transparent conducting oxides are reviewed in the August, 2000 isuue ofthe Materials Research Bulletin, Volume 25 (8) 2000, devoted tomaterials and properties of transparent conducting oxide compounds.

SUMMARY OF THE INVENTION

This invention relates to novel, transparent oxide semiconductor (TOS)thin film transistors (TFT's) and the process for their deposition,where the transparent oxide semiconductor (TOS) is selected from thegroup consisting of zinc oxide (ZnO), indium oxide (In₂O₃), tin oxide(SnO₂), or cadmium oxide (CdO) semiconductor and combinations thereof.The TFT structure described includes the TOS with conducting electrodes,commonly referred to as a source and a drain, for injecting a currentinto the TOS and a capacitance charge injection scheme for controllingand/or modulating the source-drain current. The semiconductor depositionprocess uses magnetron sputtering of an oxide (ZnO, In₂O₃, SnO₂, CdO) ormetal (Zn, In, Sn, Cd) target in an atmosphere with a controlled partialpressure of oxygen in an inert gas. This is a low temperature processwhich is compatible with temperature sensitive substrates andcomponents. One particularly attractive application of TOS TFT's is inthe drive circuits for displays on flexible, polymer substrates.

The process specifically involves depositing an undoped transparentoxide semiconductor in a field effect transistor, comprising a methodselected from the group consisting of:

-   -   a) physical vapor deposition of undoped TOS in an effective        partial pressure of oxygen mixed with an inert gas;    -   b) resistive evaporation of undoped TOS in an effective partial        pressure of oxygen;    -   c) laser evaporation of undoped—TOS in an effective partial        pressure of oxygen;    -   d) electron beam evaporation of undoped TOS in an effective        partial pressure of oxygen; and    -   e) chemical vapor deposition of undoped TOS in an effective        partial pressure of oxygen.        The invention also concerns a transistor comprising with an        undoped transparent oxide semiconductor. In one embodiment the        transister is on a flexible substrate and further comprises a        gate dielectric fabricated from a material selected from the        group consisting of zinc oxide, indium oxide, tin oxide, and        cadmium oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the dependence of resistivity on pO₂ for ZnO films rfmagnetron sputtered in 10 m Torr and 20 m Torr of argon and oxygen.

FIG. 2 shows the general resistance characteristic as a function of thepartial pressure of the oxygen source for ZnO films made by PVD or CVDmethods.

FIG. 3 shows the ZnO TFT test configuration.

FIG. 4 shows a ZnO TFT I-V curve for rf (radio frequency) magnetronsputtered films made with pO₂=P_(c)

FIG. 5 shows a ZnO TFT I-V curve for rf magnetron sputtered films madewith pO₂=2P_(c)

FIG. 6 shows a ZnO TFT I-V curve for rf magnetron sputtered films madewith pO₂=0.75P_(c)

FIG. 7 shows a ZnO TFT I-V curve for rf magnetron sputtered films madewith pO₂=0.08P_(c)

FIG. 8 shows a ZnO TFT I-V curve for rf magnetron sputtered films madewith pO₂=20P_(c)

FIGS. 9 (a) and (b) show a I-V curve for ZnO TFT fabricated on aflexible substrate. FIG. 9 (a) shows a curve of Id versus Vd varying thegate voltage from 0V to 20V in steps of 1V. FIG. 9( b) shows Id versusgate coltage as Vd=20V. In this transistor, W=400 μm and L=40 μm.

FIG. 10 shows optical images of a TFT comprised of only ZnO.

FIG. 11 shows I-V curve for a TFT comprised of only ZnO.

FIG. 12 shows I-V curve for a indium oxide TFT fabricated with pO₂ nearP_(c).

FIG. 13 shows a graph of transistor current (I_(d)) versus drain voltage(V_(d)) curves for gate voltages between zero and three (3) volts andV_(d) between 0 and 3 V.

DETAILED DESCRIPTION

While most electronic devices are fabricated today on rigid substrates,such as single crystalline Si or glass, there is a growing interest indevices on plastic or flexible substrates, particularly because theywould be more mechanically robust, lighter weight, and potentiallycheaper to manufacture by roll-to-roll processing. However, plasticsubstrates, such as polyethylene terephthalate (e.g., Mylar®, E.I.DuPont de Nemours Inc., and Wilmington, Del.) limit device processing tobelow 100 C. One consequence is that electronics based on Si, evenamorphous Si, is incompatible with temperature-sensitive plasticsubstrates. This has fueled a broad interest in organic semiconductorsas a low temperature class of alternative materials. However, mostorganic semiconductors generally have inferior electronic properties,compared to amorphous Si, for device application. Further, organicmaterials commonly degrade in normal atmospheric conditions, requiringprotection strategies. In contrast a stable inorganic semiconductor withprocessing compatible with temperature-sensitive substrates, andelectronic properties equivalent to amorphous Si would enableelectronics for a variety of flexible substrates. For this applicationthin film transistors based on novel sputtered transparent oxidesemiconductors can be made with excellent electronic properties onflexible substrates. The TOS is also transparent in the visible part ofthe electromagnetic spectrum. This may be of particular advantage (1) inelectronic display applications.

As an example, magnetron sputtering is used to form the ZnOsemiconductor layer. Using a unique range of deposition conditions, withno intentional substrate heating (compatible with low temperatureplastic substrates), novel ZnO layers were made that werepolycrystalline (X-ray diffraction) with good electron transportproperties. The ZnO layers are suitable for application assemiconductors in TFTs.

The good transport characteristics of the ZnO semiconductor of thisinvention, and prototypical of these TOS, include high electricalresistivity, for low device ‘off’ current combined with high chargecarrier mobility for high ‘on’ device current. In the sputtered ZnO thinfilms of the present invention, the electrical resistivity is controlledby metering the partial pressure of oxygen during deposition. A novelaspect of our preparation of ZnO was the discovery that sputteringconditions favorable for achieving low ZnO film stress were alsofavorable for high transconductance and high on/off current ratio in ZnOTFT devices made at room temperature. It is believed the reason is thatlow stress ZnO films have fewer defects and a favorable electronicstructure, which promote higher charge carrier mobility. Consequently,the ZnO films of the present invention exhibit better TFT deviceperformance.

In one embodiment of this invention, the source, drain, and gateelectrodes are resistance evaporated Al about 100 nm thick. The ZnOsemiconductor is about 100 nm thick layer made by rf magnetronsputtering in a mixture of Ar and O₂ gases. The gate insulator is Al₂O₃,e-beam vapor-deposited with thickness between 100 nm and 300 nm. Thesubstrates are polyethylene terephthalate (PET and Kapton® polyimide,E.I. DuPont de Nemours Inc., Wilmington, Del.). All depositions werecarried out, while maintaining the substrate at or near roomtemperature.

A thin film transistor (TFT) is an active device, which is the buildingblock for electronic circuits that switch and amplify electronicsignals. Attractive TFT device characteristics include a low voltage toturn it on, a high transconductance or device current/(gate) controlvoltage ratio, and a high ‘on’ (Vg>0) current to ‘off’ (Vg≦0) currentratio. In a typical TFT structure of this invention, the substrate ispaper or polymer, such as PET, PEN, Kapton, etc. Source and drainconducting electrodes are patterned on the substrate. The TOS is thendeposited, followed by a gate insulating layer such as SiO₂ or Al₂O₃.Finally, a gate conducting electrode is deposited on the gate insulatinglayer. One of skill in the art will recognize, this is one of manypossible TFT fabrication schemes. In the operation of this device, avoltage applied between the source and drain electrodes establishes asubstantial current flow only when the control gate electrode isenergized. That is, the flow of current between the source and drainelectrodes is modulated or controlled by the bias voltage applied to thegate electrode. The relationship between material and device parametersof the TOS TFT can be expressed by the approximate equation,I _(sd)=(W/2L) Cμ(V _(g))²where I_(sd) is the saturation source-drain current, C is the geometricgate capacitance, associated with the insulating layer, W and L arephysical device dimensions, μ is the carrier (hole or electron) mobilityin the TOS, and V_(g) is the applied gate voltage. Ideally, the TFTpasses current only when a gate voltage of appropriate polarity isapplied. However, with zero gate voltage the “off” current betweensource and drain will depend on the intrinsic conductivity,σ=nqμof the TOS, where n is the charge carrier density, and q is the charge,so that(I _(sd))=σ(Wt/L) V _(sd @Vg=)0Here t is the TOS layer thickness and V_(sd) is the voltage appliedbetween source and drain. Therefore, for the TFT to operate as a goodelectronic switch, e.g. in a display, with a high on/off current ratio,the TCOS semiconductor needs to have high carrier mobility but verysmall intrinsic conductivity, or equivalently, a low charge carrierdensity. On/off ratios >10³ are desirable for practical devices.

Specifically, when undoped ZnO thin films are dc or rf magnetronsputtered from a Zn or ZnO target in a partial pressure of oxygen, pO₂,the bulk resistivity (R) changes abruptly from strongly semi-conducting(R˜10⁻² ohm cm), to nearly insulating (R˜10⁶−10⁸ ohm cm), withincreasing pO₂. This dependence of R on pO₂ for ZnO films rf magnetronsputtered from an undoped ZnO target is shown in FIG. 1. (The dependenceof R on pO₂ will be similar for indium oxide, tin oxide, and cadmiumoxide thin films). The sputtering system consisted of a cryo-pumpedstainless steel vacuum chamber (about 25 inch diameter×15 inch high)with a water-cooled stationary table for substrates. The target diameterwas 6.5 inches, the substrate-to-target distance was about 3 inches, andrf (13.56 MHz) power was coupled to the target through a standardimpedance matching network. The vendor analysis of the target indicatedit contained impurities of As, Fe, Cd, Cu, Ca, Mn, Na, Pb in amounts<20ppm. For the ZnO films whose resistivities are given in FIG. 1, there isa critical oxygen partial pressure, P_(c), for which the change inresistivity, ΔR_(c) in the vicinity of P_(c) is very large and abrupt.P_(c) is defined as the oxygen partial pressure corresponding to the midpoint of the abrupt rise in resistivity. Specifically, ΔR_(c) increasedby >10⁴ ohm cm for pO₂ between P_(c)/2 and 2P_(c). For FIG. 1, thecritical pressure, P_(c) is approximately 10⁻⁵ Torr. This characteristicof an abrupt, large change in R versus pO₂, occurring at a criticaloxygen partial pressure P_(c) is a general result, as sketched in FIG.2, for ZnO films and other TOS films prepared by any vapor depositionmethod, chemical or physical, that requires a source of oxygen for thesynthesis. Physical vapor deposition (PVD) principally involves allforms of sputtering (rf, dc, magnetron, diode, triode, ion-beam) andevaporation (resistive, laser, electron beam). Commonly PVD of TOSrelies on a solid or molten source of the corresponding metal or metaloxide. Chemical vapor deposition (CVD) requires chemical vapor transportand chemical reaction for film formation. Reactants are commonlygaseous, and examples of reaction types include pyrolysis, reduction,oxidation, disproportionation, and compound formation. CVD processesinclude low-pressure (LPCVD), plasma-enhanced (PECVD), atomic layerchemical vapor deposition (ALCVD, also known as atomic layer deposition,ALD), and laser-enhanced (LECVD) methods.

Independent of preparation method, P_(c) defines oxide growthconditions, for which the arrival rate of atomic oxygen just matches thearrival rate of atomic Zn, In, Sn, or Cd to form the stoichiometricoxide, e.g. ZnO, with semi-insulating resistivity, i.e. ˜10⁸ ohm cm.Consequently, only a small deviation from stoichiometry e.g.,Zn_(1.0001) O_(1.0000), will reduce the resistivity by orders ofmagnitude, since 0.01% excess Zn corresponds to ˜10¹⁹ free electrons(two electrons per interstitial Zn ion) or a resistivity ˜1 ohm cm forμ˜1 cm²/V−s. Therefore, in the vicinity of P_(c), only a very smallchange in pO₂ will cause a large, abrupt change in resistivity,independent of the preparation method.

However, the actual value of P_(c) will depend on specific depositionconditions and the specific oxide as well as the physical and dynamiccharacteristics of the deposition system. Also, the actual magnitude ofthe resistance change, ΔR_(c) in the vicinity of P_(c), will depend onthe level of impurities (dopants) incorporated into the oxide film. Alower impurity level will increase the magnitude of ΔR_(c), whereas ahigher concentration of impurities will reduce it. But the generalresistance characteristic will be system invariant, so that one skilledin the art of vapor deposition can find P_(c) for that particular systemused to make undoped TOS films.

The field effect transistors of the present invention based on anominally undoped TOS must be deposited under an effective partialpressure of oxygen using physical vapor deposition or chemical vapordeposition, preferably rf magnetron sputtering. An effective partialpressure of oxygen is a range of oxygen partial pressure about thecritical partial pressure such that the electrical resistivity isintermediate between a low, nearly-conductive value observed for verylow oxygen partial pressures and a high, nearly-insulating value valueobserved for high oxygen partial pressures. The best performance (highchannel current and high device on/off ratio) occurs when a TOS is madeby vapor deposition in the preferable range of oxygen partial pressure,0.1 P_(c)<pO₂<10 P_(c), and more preferably in the range, 0.5P_(c)<pO₂<2 P_(c). The following examples of magnetron sputtered ZnOthin film transistors and an In₂O₃ TFT illustrate this effect.Conditions for ZnO preparation with pO₂ in the range 0.1 P_(c)<pO₂<10P_(c), where P_(c)≈10⁻⁵ Torr were chosen for sputtering in Examples 1-3.Examples 4 and 5 illustrate that sputtering outside the preferred pO₂produces TFTs with inferior properties. Example 6 illustrates thestructure and properties of a ZnO TFT fabricated on a flexiblesubstrate. Example 7 illustrates properties of a ZnO TFT comprised ofconducting ZnO source, drain, and gate electrodes, semiconducting ZnOchannel, and a ZnO dielectric. Example 8 describes properties of anindium oxide TFT made near the critical oxygen partial pressure.

The general structure of the ZnO and In₂O₃ field effect transistor ofthese examples is shown in FIG. 3. TFTs were fabricated on heavily dopedn-type Si substrates with a thermal oxide layer about 100 nm thick onone side. Ti—Au source and drain electrodes (10 nm Ti followed by 100 nmAu), 200 μm wide with a 20 μm gap were deposited and patterned directlyon the thermal silicon oxide layer by traditional photolithography.Ti—Au was also deposited on the back-side of the Si as a common gateelectrode, and ZnO or In₂O₃ about 100 nm thick was then sputteredbetween source and drain electrodes using a shadow mask.

The TFT structure described herein includes a transparent oxidesemiconductor with conducting electrodes, commonly referred to as asource and a drain, for injecting a current into the oxide semiconductorand a capacitance charge injection scheme for controlling and/ormodulating the source-drain current. The semiconductor depositionprocess uses magnetron sputtering of an oxide or metal target in anatmosphere with a controlled partial pressure of oxygen in an inert gas.This is a low temperature process which is compatible with temperaturesensitive substrates and components. One particularly attractiveapplication of TOS TFT's is in the drive circuits for displays onflexible, polymer substrates. TOS transistors and/or transistor arraysare useful in applications including, but not limited to, flat paneldisplays, active matrix imagers, sensors, rf price labels, electronicpaper systems, rf identification tags and rf inventory tags.

The TFT structure described herein is applicable to flexible substrates.The flexible substrate may be a polymer film such as, but not limitedto, polyethylene terephthalate (PET), polyethylene naphthalate (PEN),polyethersulphone (PES) and polycarbonate (PC). Flexible substrates canalso be thin metal foils such as stainless steel provided they arecoated with an insulating layer to electrically isolate the thin filmtransistor.

By control of the oxygen partial pressure during deposition, it ispossible to control the electrical conductivity of the undoped metaloxide such that the metal oxide can be an insulator, semiconductor orconductor. Thus by varying the oxygen partial pressure, all elements ofa thin film transistor, semiconductor, conductors (source, drain andgate) and insulators (gate dielectric) can be made from the same oxidematerial but deposited under different conditions.

EXAMPLES Example 1

Using the transistor configuration shown in FIG. 3, a ZnO thin filmsemiconductor was rf magnetron sputtered at room temperature to depositbetween source and drain electrodes, using a shadow mask. The ZnO targetwas 6.5 inch diameter and the rf power for sputtering was 100 W. Thetotal gas pressure during sputtering was 20 m Torr, comprised of 1×10⁻⁵Torr of oxygen, or pO₂=P_(c), with the balance being argon. The ZnO filmthickness, determined optically, was 849 A for a sputtering time of 500sec. FIG. 4 is a set of corresponding drain current (I_(d)) versus drainvoltage (V_(d)) transistor curves for gate voltages (V_(g)) between zeroand 50 V. For this device, the field effect mobility (μ_(FE)) from thelinear current-voltage characteristics was determined to be 1.2 cm²/V−swith an on/off ratio equal to 1.6×10⁶. This on/off ratio corresponds tothe ratio of source-drain current with 50 V and 0 V bias on the gateelectrode while applying 10V between source and drain electrodes.

Example 2

Using the transistor configuration shown in FIG. 3, a ZnO thin filmsemiconductor was rf magnetron sputtered at room temperature to depositbetween source and drain electrodes, using a shadow mask. The ZnO targetwas 6.5 inch diameter and the rf power for sputtering was 100 W. Thetotal gas pressure during sputtering was 20 m Torr, comprised of 2×10⁻⁵Torr of oxygen, or pO₂=2P_(c), with the balance being argon. The ZnOfilm thickness, determined optically, was 677 A for a sputtering time of500 sec. FIG. 5 is a set of corresponding drain current (Id) versusdrain voltage (Vd) transistor curves for gate voltages (Vg) between zeroand 50 V. For this device, the field effect mobility (μ_(FE)) from thelinear current-voltage characteristics was determined to be 0.3 cm²/V−swith an on/off ratio equal to 1.0×10⁵.

Example 3

Using the transistor configuration shown in FIG. 3, a ZnO thin filmsemiconductor was rf magnetron sputtered at room temperature to depositbetween source and drain electrodes, using a shadow mask. The ZnO targetwas 6.5 inch diameter and the rf power for sputtering was 100 W. Thetotal gas pressure during sputtering was 20 m Torr, comprised of0.75×10⁻⁵ Torr of oxygen, or pO₂=0.75 P_(c), with the balance beingargon. The ZnO film thickness, determined optically, was 897 A for asputtering time of 500 sec. FIG. 6 is a set of corresponding draincurrent (I_(d)) versus drain voltage (V_(d)) transistor curves for gatevoltages (V_(g)) between zero and 50 V. For this device, the fieldeffect mobility (μ_(FE)) from the saturation current-voltagecharacteristics was determined to be 6.8 cm²/V−s with an on/off ratioequal to 1×10³.

Example 4

Using the transistor configuration shown in FIG. 3, a ZnO thin filmsemiconductor was rf magnetron sputtered at room temperature to depositbetween source and drain electrodes, using a shadow mask. The ZnO targetwas 6.5 inch diameter and the rf power for sputtering was 100 W. Thetotal gas pressure during sputtering was 20 m Torr, comprised of0.8×10⁻⁶ Torr of oxygen, or pO₂=0.08 P_(c), with the balance beingargon. The value pO₂=0.08 P_(c) is outside our preferred range of oxygenpartial pressure. The ZnO film thickness, determined optically, was 1071A for a sputtering time of 465 sec. FIG. 7 shows a set of thecorresponding drain current (Id) versus drain voltage (Vd) curves forgate voltages (Vg) of zero, 30 V and 40 V. This device does not have theperformance characteristics of a transistor. There is negligibly smallmodulation of the current by application of a gate voltage and the ratioof the device current with no gate and either 30 or 40 V gate isunacceptably close to one. The device acts more like a resistor.

Example 5

Using the transistor configuration shown in FIG. 3, a ZnO thin filmsemiconductor was rf magnetron sputtered at room temperature to depositbetween source and drain electrodes, using a shadow mask. The ZnO targetwas 6.5 inch diameter and the rf power for sputtering was 100 W. Thetotal gas pressure during sputtering was 20 m Torr, comprised of 2×10⁻⁴Torr of oxygen, or pO₂=20 P_(c), with the balance being argon. The valuepO₂=20 P_(c) is outside our preferred range of oxygen partial pressure.The ZnO film thickness, determined optically, was 1080 A for asputtering time of 465 sec. FIG. 8 shows a set of the correspondingdrain current (Id) versus drain voltage (Vd) curves for gate voltages(Vg) of zero to 50 V. The device I-V curve is characteristic of a thinfilm transistor, however the drain current is quite small. For thisdevice, the field effect mobility (μ_(FE)) from the linearcurrent-voltage characteristics was determined to be 5×10⁻⁵ cm²/V−s withan on/off ratio equal to about 700. Both the mobility and the on/offratio are much smaller than for TFT devices made within the preferredrange of pO₂.

Example 6

As an example of a ZnO TFT on a flexible substrate, transistors werefabricated on DuPont Pyralux® (Cu-coated) polyimide. Cu source and drainwere lithographically patterned using DuPont Riston® uv-imaged through aphototool, followed by sputtering 100 nm thick ZnO semiconductor. (TheZnO sputtering conditions were identical to those in Example 1). Afluoropolymer dielectric (relative dielectric constant, ε=8.7) was thenlaminated at 120° C. over the semiconductor active region, and Al gateswere thermally evaporated using a shadow mask. FIGS. 9 (a) and (b) showsthe performance of these flexible transistors, which have μ˜0.4 cm²/V−sand on/off ratios larger than 10⁴.

Example 7

By tailoring the resistivity of ZnO films from semiconducting tosemi-insulating, as shown in FIG. 1, a transparent thin film transistorwas fabricated using only ZnO. The substrates were glass andpolyethylene terephthalate (PET). Source-drain electrodes of conductingZnO were first grown by sputtering at 100 W from a ZnO target in 10 mTorr of Ar without oxygen. The semiconducting channel layer, 100 nmthick, was then sputtered at 20 m Torr Ar and 1×10⁻⁵ Torr of O₂. Thenext layer was a semi-insulating ZnO for the gate dielectric, 500 nmthick, made by sputtering a ZnO target in a 50% mixture of Ar+O₂ at atotal pressure of 10 m Torr. Finally the ZnO gate electrode wassputtered at the same conditions used for the source-drain electrodes.As shown in FIG. 10, this structure is optically transparent, allowingeasy reading of the caption, “ZnO TFT” beneath the transistor. Thecurrent-voltage characteristic in FIG. 11 illustrates that thesource-drain current can be modulated by an application of a gatevoltage.

Example 8

Using the transistor configuration shown in FIG. 3, an indium oxide thinfilm semiconductor was rf magnetron sputtered at room temperature todeposit between source and drain electrodes, using a shadow mask. Theindium oxide target was 6.5 inch diameter and the rf power forsputtering was 100 W. The total gas pressure during sputtering was 12 mTorr, comprised of 2 m Torr of oxygen, or pO₂ close to P_(c), with thebalance of 10 m Torr argon. The indium oxide film thickness, determinedoptically, was 1285 A for a sputtering time of about 33 min. FIG. 12 isa set of corresponding drain current (I_(d)) versus drain voltage(V_(d)) transistor curves for gate voltages (V_(g)) between −20 V and 10V. For this device, the field effect mobility (μ_(FE)) from the linearcurrent-voltage characteristics was determined to be 17 cm²/V−s with anon/off ratio equal to about 2×10². This on/off ratio can likely beimproved by use of a higher gate voltage and optimization of sputteringconditions.

Example 9

This example illustrates low voltage and high current operation in a ZnOTFT on an aluminum oxide gate dielectric. The substrate, which alsoserved as the gate electrode, was a heavily doped (with Phosphorous),n-type silicon wafer, 1-inch×1-inch×475 microns thick. One side of thiswafer was coated with an aluminum oxide gate dielectric layer byelectron-beam evaporation from a high purity, solid aluminum-oxidesource. The measured aluminum oxide film thickness was 4483 A.Aluminum-metal source and drain electrodes, about 1500 A thick, werethermally evaporated on the oxide dielectric through a shadow mask tocreate a transistor channel length 80 microns by about 800 microns wide.A shadow mask was then used to magnetron sputter ZnO semiconductor, 918A thick, in the channel between source and drain electrodes. Sputteringwas in 20 m Torr Ar and 1×10⁻⁵ Torr O₂. FIG. 13 is a set of transistorcurrent (I_(d)) versus drain voltage (V_(d)) curves for gate voltagesbetween zero and three (3) volts and V_(d) between 0 and 3 V. For thisdevice the field-effect mobility was determined to be ˜3 cm²/V−s with anon-off ratio>10³. For three volt operation the current is substantialat >1 microampere.

1. A process comprising: depositing an undoped transparent oxide semiconductor, comprising at least one oxide selected from the group consisting of zinc oxide, indium oxide, tin oxide, and cadmium oxide, in a field effect transistor, by a method selected from the group consisting of: a) physical vapor deposition of an undoped transparent oxide semiconductor in a controlled partial pressure of oxygen of 0.1 P_(c) to 10 P_(c), in an inert gas; b) resistive evaporation of an undoped transparent oxide semiconductor in a controlled partial pressure of oxygen of 0.1P_(c) to 10P_(c); c) laser evaporation of an undoped transparent oxide semiconductor in a controlled partial pressure of oxygen of 0.1 P_(c) to 10 P_(c), in an inert gas; d) electron beam evaporation of an undoped transparent oxide semiconductor in a controlled partial pressure of oxygen of 0.1 P_(c) to 10 P_(c), in an inert gas; and e) chemical vapor deposition of an undoped transparent oxide semiconductor in a controlled partial pressure of oxygen of 0.1 P_(c) to 10 P_(c), in an inert gas, wherein P_(c) is oxygen critical pressure.
 2. The process of claim 1 wherein the physical vapor deposition is rf magnetron sputtering.
 3. The process of claim 1 wherein the physical vapor deposition is dc magnetron sputtering.
 4. The process of claim 1 wherein the physical vapor deposition is diode sputtering.
 5. The process of claim 1 wherein the physical vapor deposition is triode sputtering.
 6. The process of claim 1 wherein the physical vapor deposition is ion beam sputtering.
 7. The process of any of claims 1(a), 2, 3, 4, 5 or 6 wherein deposition is by physical vapor deposition and wherein the inert gas is selected from the group consisting of helium, neon, argon, krypton, and xenon.
 8. The process of claim 1(e) wherein the chemical vapor deposition is low pressure chemical vapor deposition.
 9. The process of claim 1(e) wherein the chemical vapor deposition is plasma-enhanced chemical vapor deposition.
 10. The process of claim 1(e) wherein the chemical vapor deposition is laser-enhanced chemical vapor deposition.
 11. The process of claim 1(e) wherein the chemical vapor deposition is atomic layer chemical vapor deposition.
 12. The process of claim 1 wherein the effective partial pressure of oxygen is between 0.5 and 2 times the critical pressure. 